1. Field of the Invention
The present invention generally relates to an electric vehicle including a front electric motor for driving a front wheel and a rear electric motor for driving a rear wheel. Particularly, the present invention relates to control of the front electric motor and the rear electric motor.
2. Description of the Related Art
Conventionally, as electric vehicles, there are electric vehicles including only an electric motor as a driving power source, hybrid vehicles including an engine and a motor, etc. In these electric vehicles, a single motor is provided to drive one of either front wheel(s) or rear wheel(s), a front motor and a rear motor are provided to drive front wheel(s) and rear wheel(s), respectively, or motors are provided to correspond to right and left front wheels and right and left rear wheels, respectively, in one-to-one correspondence (wheel-in type).
For example, Japanese Patent No. 3678053 discloses a technique in which in an electric vehicle configured such that a front motor and a rear motor drive front wheels and rear wheels, respectively, a speed reduction ratio from the front motor to the front wheels is made different from a speed reduction ratio from the rear motor to the rear wheels, and the vehicle is actuated by using only the motor corresponding to a lower speed reduction ratio under a smaller driving load, thereby lessening a transmission loss of a driving power.
However, in the above conventional example in which the electric vehicle is configured to be actuated by using only one of the motors depending on the driving load, if a wheel slip occurs in a slippery road surface condition such as a snowy road surface or a sandy road surface condition in which a friction coefficient is very low, the driving load is reduced, and as a result, the electric vehicle may possibly be actuated by only one of the motors. In other words, four-wheel-drive capability of the vehicle cannot be effectively used, and driving mobility of the vehicle degrades.
In addition, if the driver is depressing an accelerator pedal to move the vehicle forward, the two wheels which are the front wheels or the rear wheels are slipping, but the remaining two wheels are locked, which make the driver feel discomfort.